In the conventional railroad braking system, which is derived from the Westinghouse airbrake, brakes are controlled by air pressure in a brake air line, brakepipe, which is connected between individual railway vehicles in a consist of coupled railway vehicles. Pressure in the brake air line is generally controlled by a lead locomotive of the consist. Fail-safe operation is provided by designing the system so that the brake air line is pressurized to release the brakes, and depressurized to apply brakes. Emergency braking is provided by rapidly venting air from the brakepipe. If the brake air line fails so that its pressure drops, brakes are automatically applied.
Some time is required for a pressure decrement, initiated by the lead locomotive, to propagate down the line of railway vehicles. A pressure decrement for a service brake application requires about one minute to propagate down the length of a mile-long freight train. A pressure decrement for an emergency brake application requires about one half of a minute for a mile-long train.
In order to provide more rapid application of brakes, for either service or emergency application, an exhaust valve may be provided in a portion of the train. It may be in a remote locomotive of the train or in an end-of-train unit. The remote unit or end of train unit is usually controlled by radio signals from the locomotive, or it may be controlled by electrical trainlines which are connected between the individual railway vehicles of the train. The trainlines carry electrical signals from the lead locomotive to the remote unit.
The exhaust valve has a control reservoir which, during normal operation, contains air which is at the pressure of the brake air line. A prior art valve has a piston in a cylinder and has a diaphragm to prevent leakage between the piston and the cylinder. The piston has air on one side which is at the pressure of the brakepipe, and air on a second side which is at the pressure of the control reservoir. The piston is connected to a brakepipe exhaust valve which is closed when air on each side of the piston is at the same pressure.
When the remote unit or end of train unit receives a signal from the lead locomotive to exhaust air from the brake air line, it vents air from the control reservoir. This causes the piston to move and the brakepipe exhaust valve is opened.
The diaphragm has an annular fold which is directed toward the second side of the piston to permit the piston to move, while sealing between the piston and the cylinder. In normal operation of the exhaust valve, the air pressure on the first side of the piston is always equal to or greater than the air pressure on the second side of the piston, so the annular fold is able to withstand the pressure difference.
However, in a situation in which the brake air line fails so that its pressure drops, and no signal is received by the remote unit or end of train unit to reduce the pressure in the control chamber, then the pressure on the second side of the piston becomes greater than the pressure on the first side of the piston. In this case, the annular fold of the diaphragm may become jammed into the gap between the piston and the cylinder. The system must then be serviced before it can be used again.